<code>validateConnection(cellViewS, magnetS, cellViewT, magnetT, end, linkView)</code> - 
                                decide whether to allow or disallow a connection between the source view/magnet (<code>cellViewS/magnetS</code>) 
                                and target view/magnet (<code>cellViewT/magnetT</code>). <code>end</code> is either <code>&quot;source&quot;</code> or <code>&quot;target&quot;</code> and tells which end of
                                the link is being dragged. This is useful for defining whether, for example, a link starting in a port POut of element A can lead to a port PIn of elmement B. By default, all linkings are allowed.